Stackable ball grid array package

ABSTRACT

A stackable FBGA package is configured such that conductive elements are placed along the outside perimeter of an integrated circuit (IC) device mounted to the FBGA. The conductive elements also are of sufficient size so that they extend beyond the bottom or top surface of the IC device, including the wiring interconnect and encapsulate material, as the conductive elements make contact with the FBGA positioned below or above to form a stack. The IC device, such as a memory chip, is mounted upon a first surface of a printed circuit board substrate forming part of the FBGA. Lead wires are used to attach the IC device to the printed board substrate and encapsulant is used to contain the IC device and wires within and below the matrix and profile of the conductive elements.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Cross Reference to Related Applications: This application is acontinuation of application Ser. No. 09/416,249, filed Oct. 12, 1999,pending, which is a divisional of application Ser. No. 09/072,101, filedMay 4, 1998, now U.S. Pat. No. 6,072,233, issued Jun. 6, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention relates generally to packagingsemiconductor devices and, more particularly, the present inventionrelates to fine ball grid array packages that can be stacked to formhighly dense components.

[0003] Ball grid array (BGA) semiconductor packages are well known inthe art. BGA packages typically comprise a substrate, such as a printedcircuit board, with a semiconductor die mounted on the top side of thesubstrate. The semiconductor die has a multitude of bond padselectrically connected to a series of metal traces on the top side ofthe printed circuit board. The connection between the bond pads and themetal traces is provided by wire bonds electrically and mechanicallyconnected between the two. This series of metal traces is connected to asecond series of metal traces on the underside of the printed circuitboard through a series of vias. The second series of metal traces eachterminate with a connect contact pad where a conductive element isattached. The conductive elements can be solder balls or conductivefilled epoxy. The conductive elements are arranged in an array patternand the semiconductor die and wire bonds are encapsulated with a moldingcompound.

[0004] As chip and grid array densities increase, the desire inpackaging semiconductor chips has been to reduce the overall height orprofile of the semiconductor package. The use of BGAs has allowed forthis reduction of profile as well as increased package density. Densityreduction has been achieved by utilizing lead frames, such as lead-overchips, in order to increase the densities as well as to branch out intobeing able to stack units one on top another.

[0005] One example of a lead chip design in a BGA package is shown inU.S. Pat. No. 5,668,405, issued Sep. 16, 1997. This patent discloses asemiconductor device that has a lead frame attached to the semiconductorchip. Through holes are provided that allow for solder bumps to connectvia the lead frame to the semiconductor device. This particularreference requires several steps of attaching the semiconductor deviceto the lead frame, then providing sealing resin, and then adding a basefilm and forming through holes in the base film. A cover resin is addedbefore solder bumps are added in the through holes to connect to thelead frame. This particular structure lacks the ability to stack devicesone on top another.

[0006] U.S. Pat. No. 5,677,566, issued Oct. 14, 1997, and commonlyassigned to the assignee of the present invention, discloses asemiconductor chip package that includes discrete conductive leads withelectrical contact bond pads on a semiconductor chip. The lead assemblyis encapsulated with a typical encapsulating material and electrodebumps are formed through the encapsulating material to contact theconductive leads. The electrode bumps protrude from the encapsulatingmaterial for connection to an external circuit. The semiconductor chiphas the bond leads located in the center of the die, thus allowing theconductive leads to be more readily protected once encapsulated in theencapsulating material. Unfortunately, this particular assembly taughtin the '566 patent reference also lacks the ability to stack onesemiconductor device on top another.

[0007] Attempts have been made to stack semiconductor devices in threedimensional integrated circuit packages. One such design is disclosed inU.S. Pat. No. 5,625,221, issued Apr. 29, 1997. This patent discloses asemiconductor package assembly that has recessed edge portions thatextend along at least one edge portion of the assembly. An upper surfacelead is exposed therefrom and a top recess portion is disposed on a topsurface of the assembly. A bottom recess portion is disposed on thebottom surface of the assembly such that when the assembly is used infabricating a three-dimensional integrated circuit module, the recessededge portion accommodates leads belonging to an upper semiconductorassembly to provide electrical interconnection therebetween.Unfortunately, the assembly requires long lead wires from thesemiconductor chip to the outer edges. These lead wires add harmfulinductance and unnecessary signal delay and can form a weak link in theelectrical interconnection between the semiconductor device and theouter edges. Further, the device profile is a sum of the height of thesemiconductor die, the printed circuit board to which it is bonded, theconductive elements, such as the solder balls, and the encapsulant thatmust cover the die and any wire bonds used to connect the die to theprinted circuit board. So, reducing the overall profile is difficultbecause of the geometries required in having the lead pads on thesemiconductor chip along the outer periphery with extended lead wiresreaching from the chip to the outer edges.

[0008] Another stacked arrangement of semiconductor devices on asubstrate interconnected by pins is illustrated in U.S. Pat. Nos.5,266,912 and 5,400,003. However, the height of the stacked package islimited by the length of the pin connections between the individualmulti-chip modules or printed circuit boards.

[0009] Accordingly, what is needed is a ball grid array package thatallows stacking of packages on one another. This stackable package wouldhave a lower profile than otherwise provided in the prior art and wouldreduce the number of steps in the assembly of the package.

SUMMARY OF THE INVENTION

[0010] According to the present invention, a stackable fine ball gridarray (FBGA) package is disclosed that allows the stacking of one arrayupon another. This stackable FBGA package is configured such thatconductive elements are placed along the outside perimeter of asemiconductor device (integrated circuit (IC) device) mounted to theFBGA. The conductive elements also are of sufficient size so that theyextend beyond the bottom or top surface of the IC device. Wireinterconnect connects the IC device in a way that does not increase theoverall profile of the package. Encapsulating material protects both theIC device and the wire interconnect as the conductive elements makecontact with the FBGA positioned below or above to form a stack. The ICdevice, such as a memory chip, is mounted upon a first surface of aprinted circuit board substrate forming part of the FBGA. Lead wires, orwire interconnect, are used to attach the IC device to the printedcircuit board substrate and an encapsulant is used to contain the ICdevice and wires within and below the matrix and profile of theconductive elements.

[0011] Additionally, certain pins on the FBGA in the stack require anisolated connection to the PC board. An example of such a requirement iswhen an activation signal for a particular IC device within the stackmust be sent solely to that device and not to any of the other deviceswithin the stack. This isolated connection connects to an adjacent ballon a different FBGA stack above or below that particular isolatedconnection since in common pin layouts of the devices are stackedtogether, and each device requires an isolated connection to the PCboard. This provides for a stair step connection from the bottom of theFBGA stacked array to the top that allows each device, from the bottomone to the top one, to have an isolated connection from each other. Thisallows IC devices to be stacked one upon the other while maintaining aunique pin out for each pin required in the stack.

[0012] Further, the FBGA of the present invention keeps the wire lengthsbetween the IC device and the conductors of the PC board to a minimumfor the control of the impedance of the conductors.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0013]FIG. 1 depicts a schematic cross-sectional representation of astacked array of FBGAs according to the present invention;

[0014]FIG. 2 depicts a top plan view of a representative circuit boardas used in the array of FIG. 1;

[0015]FIG. 3 depicts a perspective view of a printed circuit boardhaving traces connected one to another with vias and contact throughholes;

[0016]FIG. 4 depicts a perspective view of a pair of different printedcircuit boards having an electrical connection extending from onelocation on one board to another location on the second board;

[0017]FIG. 5 depicts a perspective view of multiple PC boardsinterconnected in a manner according to the present invention;

[0018]FIG. 6 is an alternative embodiment of a stackable array accordingto the present invention;

[0019]FIG. 7 depicts another embodiment where the ball grid array matrixextends below the semiconductor device;

[0020]FIG. 8 depicts a bottom plan view of an FBGA device found in FIG.1;

[0021]FIG. 9 is a schematic diagram of a view of a printed circuit boardhaving a mounted IC with wire leads attaching the bond pads of the IC tothe bond pads of the printed circuit board;

[0022]FIG. 10 is a cross-sectional view of a portion of a printedcircuit board illustrating the pin and connection therebetween;

[0023]FIG. 11 is a cross-sectional view of portions of printed circuitboards illustrating the pins and connections therebetween; and

[0024]FIG. 12 is a block diagram of an electronic system incorporatingthe FBGA module of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Referring to drawing FIG. 1, illustrated in a cross-sectionalview is a plurality of fine ball grid array (FBGA) packages 10 in astacked arrangement. Each FBGA package 10 is stacked one upon anothervia a matrix of conductive elements or solder balls 28 having a firstheight. Each FBGA package 10 includes a substrate 12 that has conductivetraces formed both on the top surface and the bottom surface. Substrate12 may be formed from an organic epoxy-glass resin base material, suchas bismaleimide-triazin (BT) resin or FR-4 board, but is not limitedthereto. Other carrier substrate materials well known to those skilledin the art may also be utilized instead, such as, for example, either aceramic or silicon substrate.

[0026] FBGA package 10 further comprises an integrated circuit orsemiconductor die 14 attached to a die attach pad 16 formed on the uppersurface of substrate 12. Semiconductor die 14 is attached to die attachpad 16 using a dielectric adhesive that is non-conductive and has athermal coefficient of expansion (TCE) that closely matches that of thesemiconductor die 14. The adhesive can be any type of epoxy resin orother polymer adhesives typically used for such purposes. Alternately,the die attach pad 16 may be formed of double sided, adhesively coatedtape, such as an adhesively coated Kapton™ tape or the like. Thesemiconductor die 14 is formed having a plurality of bond pads 18 thatis formed on the active surface thereof which mates with die attach pad16 of the substrate 12. Each bond pad of the plurality of bond pads 18aligns with a corresponding aperture 24 in substrate 12. Each bond padof the plurality of bond pads 18 is electrically connected to terminalpads 20 that are on the surface of substrate 12. Wire bonds 22 are usedto form the connections between the plurality of bond pads 18 on thesemiconductor die 14 and the terminal pads 20 of the substrate 12wherein the wire bonds 22 pass through an aperture 24 formed in thesubstrate 12. A portion of semiconductor die 14 where the bond pads 18are located, along with the cavity formed by aperture 24, is covered byan encapsulating material 26. Encapsulating material 26 covers or sealsbond pads 18, terminal pads 20, and wire bonds 22 to protect them fromdust, moisture, and any incidental contact. The encapsulating material26 has a second height, the second height being less than the firstheight of the conductive elements 28.

[0027] Conductive elements 28 are attached or bonded to conductivetraces 30 (see FIG. 2) of substrate 12. Conductive elements 28 may beselected from acceptable bonding substances such as solder balls,conductive or conductor-filled epoxy, and other substances known tothose skilled in the art. The conductive elements 28, which, forexample, are solder balls, may be attached, as is known in the art, bycoating the solder balls or bond areas or both with flux, placing thesolder balls 28 on the conductive traces 30 with conventional ballplacing equipment and reflowing the balls in place using an infrared orhot air reflow process. The excess flux is then removed with anappropriate cleaning agent. In this way, the solder balls 28 areelectrically and mechanically connected to the conductive leads to formthe external electrodes. Other processes may also be used to formexternal electrodes. For example, the electrodes may be “plated up”using conventional plating techniques rather than using solder balls asdescribed above. The completed FBGA packages 10 can then be attached toa printed circuit board or the like using conventional surface mountprocesses and equipment. Likewise, each FBGA package 10 can be mountedone on top another, stacked, as is illustrated in drawing FIG. 1. Solderballs 28 may have a diameter of approximately 0.6 mm with a pitch P thatis 0.80 mm. The profile for each FBGA package 10, as measured from thebottom of solder balls 28 to the top of the semiconductor die, may rangefrom 1.0 mm to 1.22 mm.

[0028] Next, as illustrated in drawing FIG. 2, is a top plan view of thebottom surface of substrate 12. This bottom surface includespass-through aperture 24 where the wire bonds (not shown) are attachedto terminal pads 20. Each terminal pad 20 is connected to a metalconductive trace 30, which further connects to a conductive element pad32. Conductive element pads 32 are placed on either side of substrate 12and are located where the conductive elements 28 of drawing FIG. 1 aremounted. Additionally, as conductive element pads 32 are placed on theopposite side of substrate 12, they provide a pass-through connectionfor the stacking of FBGA packages 10 as shown in drawing FIG. 1.Conductive traces 30 are electrically connected to conductive traces onthe opposite side (not shown) using vias 34. Conductive traces 30 may becomprised of electrically conductive material such as copper or copperplated with gold. While conductive traces 30 are illustrated in drawingFIG. 2 on the top and bottom of the substrate 12, other conductivetraces 30 (not shown) may be located in the substrate 12 along withother vias 34 therein and conductive element pads 32 in addition tothose illustrated. Depicted in drawing FIG. 3 is a perspective view of athree dimensional drawing of how conductive traces 30 may be laid out onboth the top surface and bottom surface of substrate 12. Additionally,the conductive element pads 32 are also shown to provide connection oneither side of substrate 12. Conductive traces 30 are on both sidesconnected using vias 34 as well as the conductive elements pads 32. Theconductive traces 30 are also connected to terminal pads 20. Theaperture 24 through substrate 12 may be any desired size in relation tothe semiconductor die 14 as may be necessary. Also, the substrate 12 mayhave portions thereof removed after the mounting of the semiconductordie 14 thereon.

[0029] Depicted in drawing FIG. 4 is an expanded view of thethree-dimensional arrangement of substrates 12 achieved using thepass-through holes or vias 34 in conjunction with conductive traces 30of the substrates 12 to form a stacked arrangement. A first substrate 12is provided to connect to a second substrate 42. The connection occursat conductive element pad 32 on substrate 12 and a like conductiveelement pad 32 on second substrate 42. Next, conductive element pad 32on second substrate 42 connects to a conductive trace 30 on the surfaceof second substrate 42, which then passes from one side of secondsubstrate 42 using via 34 to connect to a bond pad on the opposite sideof second substrate 42. Referring to drawing FIG. 5, depicted is themanner in which the stepping of conductive traces can continue to yetanother level. Referring to drawing FIG. 5, depicted is a thirdconductive substrate 52 placed below substrate 12 having additionalconductive element pads 32 on either side thereof that provideconnection to the adjacent substrate 12, which then, in turn, providesconnection to second substrate 42. The arrows represent the planeconnection on semiconductor packages yet to be added.

[0030] Referring to drawing FIG. 6, depicted is an alternativeembodiment of the invention where a semiconductor die 14 is mounted onthe upper surface of substrate 12. Wire bonds 22 are then used toconnect the bond pads 18 on the active surface of the semiconductor die14 to the terminal pads 20 of substrate 12. Encapsulating material 26 isthen provided to cover the semiconductor die 14, wire bonds 22, bondpads 18 and terminal pads 20. Next, conductive elements 28 are thenmounted on the upper surface of substrate 12 around the perimeter ofsemiconductor die 14. As illustrated, this arrangement allows thestacking of multiple die packages 60. It is understood that thesubstrate 12 includes circuitry and vias (not shown) as describedhereinbefore in drawing FIGS. 2 through 5.

[0031] A third embodiment of the present invention is depicted indrawing FIG. 7. Referring to drawing FIG. 7, shown in a cross-sectionaldiagram is the manner in which a semiconductor die 14 can extend near tothe peripheral edges of substrate 12. In this case, conductive elements28 are no longer outside the perimeter of semiconductor die 14. Again,wire bonds 22 interconnect bond pads 18 of the semiconductor die 14 toterminal pads 20 on substrate 12. Encapsulating material 26 is utilizedto cover the aperture 24, the bond pads 18, terminal pads 20, and wirebonds 22. This particular arrangement of the substrate 12 andsemiconductor die 14 may be used as either a bottom level or as a toplevel in a stacked array, typically, with the use of an interposer.

[0032] Referring to drawing FIG. 8, depicted is a bottom plan view of aFBGA package 10 as illustrated in drawing FIG. 1. In this example,substrate 12 has a plurality of solder balls 28 mounted along theperimeter of semiconductor die 14, which is shown in outline form. Theconductive elements 28 form a connective matrix for connecting to thetop surface of another substrate 12 or to the top surface of a carriersubstrate that provides external electrical connectivity for the module.Encapsulating material 26 covers the wire leads and bonding pads oneither substrate 12 or semiconductor die 14.

[0033] Referring to drawing FIG. 9, illustrated is a schematic diagramof a sample pin and trace layout having isolated connection pads used toconnect to the conductive elements 28. As shown, semiconductor die bondpads 18 are aligned in a row down the center of the semiconductor die14. Wire bonds 22 interconnect bond pads 18 of the semiconductor die 14to the terminal pads 20 of the substrate 12. From terminal pads 20,conductive traces 30 interconnect conductive elements 28. As can beseen, selected conductive elements 28 have no connection to any of theconductive traces 30 or terminal pads 20 on the substrate 12. Theseconductive element areas, grouped as 29 and 31, illustrate how certainconnections are isolated from that particular semiconductor die 14mounted on that particular substrate 12. These isolated conductiveelement areas 29 and 31 allow interconnection among other FBGA packages10 (not shown) stacked one on top of the other within the stackedpackage arrangement of drawing FIG. 1. The use of selected isolated pinsallows for each semiconductor die 14 within the stacked array of FBGApackages 10 to have a unique pin out for selected pins on each layer ofFBGA packages 10. For example, in a memory package of like semiconductordies 14 stacked in an array, each semiconductor die 14 requires a selectpin that is separate from all other select pins of the othersemiconductor dies 14 within the array and that connects to a unique pinin the final pin out configuration. The stackable BGA packages areuseful in many types of electronic systems including SDRAM, EDO RAM,video RAM, cache memory, and Read-Only Memory (ROM), as well asmicroprocessors, application specific integrated circuits (ASIC),digital signal processors, flash memories, electrically erasableprogrammable read only memory (EEPROM), among others.

[0034] Referring to drawing FIG. 10, a connection terminal 100 isillustrated of substrate 12 having conductive traces 30 thereon andtherein. The substrate 12 includes conductive traces 30 and an insulatormaterial therebetween, thereby providing the ability of controlling theimpedance of the conductive traces 30 having semiconductor die 14connected thereto by wire bonds 22. The connection terminals 127 includea connection pin 141 which is connected to one of the conductive traces30. Circuitry in intermediate layers of the substrate 12 extend throughapertures 24 in order to permit all connections of the connection pins141 to be effected through the top of the substrate 12. The terminalsinclude a shield 143, which is separated from the connection pin 141 byan isolation spacer 145. The isolation spacer 145 may be of anymaterial, preferably a dielectric, provided that the isolation spacer145 permits impedance matched connection through the connectionterminals 127. Impedance matching is commonly used for signal transferapplications in which the impedance between signal carrying conductorsis a predetermined value per unit length. Changes in length will resultin proportional (inverse) changes in impedance, but not changes in theimpedance expressed per unit length. The consistent impedance per unitlength, colloquially referred to as “impedance value”, results in signalmatching. This is of interest as operating frequencies exceed those atwhich unmatched circuits are effective. The use of impedance matchedconductors in the present invention of the conductive traces 30, wirebonds 22, and connection terminals 127 therefore facilitates thefabrication of circuits which are inherently impedance matched asdesired. Matched impedance is thereby able to reduce spurious signalsbetween semiconductor dies 14, reduce circuit discontinuities, and allowconnection circuitry to be designed while controlling the establishmentof critical timing paths between components, such as semiconductor dies14.

[0035] Referring to drawing FIG. 11, the connection terminals 127 permitthe stacking of the substrate 12 with connections formed by connectionpins 141.

[0036] Referring to drawing FIG. 12, depicted is an electronic system130 that includes an input device 132 and an output device 134 coupledto a processor device 136, which, in turn, is coupled to a memory device138 incorporating the exemplary stackable FBGA package 10 and variousembodiments thereof as illustrated in drawing FIGS. 1 through 9.Likewise, even processor device 136 may be embodied in a stackable arraypackage 10 comprising a microprocessor, a first level cache memory, andadditional ICs, such as a video processor, an audio processor, or amemory management processor, but not limited thereto.

[0037] There has been shown and described a novel semiconductor chippackage that is stackable and has a lower profile over that of the priorart. The particular embodiments shown in the drawings and describedherein are for purposes of example and are not to be construed to limitthe invention as set forth in the pending claims. Those skilled in theart may know numerous uses and modifications of the specific embodimentsdescribed without departing from the scope of the invention. The processsteps described may, in some instances, be formed in a different orderor equivalent structures and processes may be substituted for variousstructures and processes described.

What is claimed is:
 1. A computer system comprising: an input device; anoutput device; a processor, coupled to said input and output devices;and a memory module, coupled to said processor, comprising: a ball gridarray, comprising: a printed circuit board substrate having a firstsurface, a second surface, an aperture, said first surface including aplurality of conductive element pads, at least one conductive elementpad on said second surface and at least one terminal pad on said secondsurface; a memory semiconductor device, mounted within a first perimeterof said first surface of said printed circuit board substrate and havingat least one bond pad; at least one wire bond, connected to the at leastone bond pad and the at least one terminal pad on said second surface ofsaid printed circuit board substrate while passing through saidaperture; a material placed along said aperture, on said at least onebond pad, said at least one terminal pad, and said at least one wirebond, forming a first profile height; and a plurality of conductiveelements, mounted along a second perimeter of said second surface, saidsecond perimeter being greater than said first perimeter, and coupled tosaid at least one conductive element pad on said second surface, saidplurality of conductive elements having a second profile height greaterthan said first profile height.
 2. The computer system according toclaim 1, wherein said plurality of conductive elements includes aplurality of conductive elements, a first part of each conductiveelement of said plurality of conductive elements aligning in a firstparallel row having a first pitch spacing.
 3. The computer systemaccording to claim 2, wherein a second part of each conductive elementof said plurality of conductive elements aligns in a second parallel rowhaving a second pitch spacing.
 4. The computer system according to claim1, wherein said material has a second profile height less than saidfirst profile height.
 5. The computer system according to claim 1,wherein said at least one conductive element pad is connected to said atleast one bond pad through said printed circuit board substrate.
 6. Thecomputer system according to claim 1, wherein at least one conductiveelement of said plurality of conductive elements is isolated.